The human foot presents a complex skeletal structure having numerous joints which are in turn controlled by a complex muscular system. While reasonably close mechanical equivalents of the skeletal structure of the foot might be constructed without any great difficulty, the provision of a control system capable of manipulating such a mechanical system to duplicate the action of a human foot presents a far greater problem both from the technological and the financial standpoints.
As a result, presently available prosthetic feet typically take the form of two or more elements coupled to each other for resiliently resisted, pivotal or flexing movement relative to each other in movement approximately duplicative of the flexing movement of portions of the human foot. Many examples of such prosthetic feet exist in the prior art; however, as noted in Truesdell Pat. No. 4,652,266, very few have met with complete approval from amputees and prosthetic specialists.
The basic requirements of an acceptable prosthetic foot are that it will provide a stable support for the amputee throughout a reasonable range of activity and permit the amputee to walk with a normal stride. To achieve this normal stride, the prosthetic foot must flex during walking as the foot continually moves through the heel-strike, foot-flat and toe-off cycle. It must also, throughout this cycle, provide transverse stability, particularly at toe-off, when the entire weight of the amputee is applied to the forward portion of the prosthetic foot. Prior art prosthetic feet typically are substantially transversely inflexible, which interferes with side-to-side balancing when walking on uneven surfaces. Unlike the natural foot, the prosthetic foot does not sense or correct itself for this unevenness and an unanticipated sidewise tilting of the foot at toe-off results in an imbalance at a critical portion of the stride.
In the case of a unilateral (one-legged) amputee, it is not possible to match the stride of the prosthetic foot with that of the natural foot unless the weight transfer from the points of support of the foot duplicate each other in the prosthetic foot and the natural foot. This requirement has not been fully appreciated by the prior art, particularly in duplicating the geometrical relationship between the amputee's center of gravity and points of support throughout heel-strike, foot-flat and toe-off. Unless, for example, the heel-strike of the prosthetic foot occurs at the same point in the stride cycle as that of the natural foot, the gait will be uneven. Similarly unless the transition of weight to the forward portion of the prosthetic foot--initiation of toe-off--does not match that of the natural foot, a hitch in stride or lurch will occur.
The present invention is directed to a prosthetic foot which presents a three-point balance system matching that of a natural foot.